1. Technical Field
The invention relates to the casting of polymeric film, and to an improved process and apparatus therefor.
2. Background Art
In Alles and Saner, U.S. Pat. No. 2,627,088, there is disclosed a process of preparing photographic film which includes casting a molten highly polymeric polyester into an unsupported sheet of film, the polyester being also capable of being spun into filaments, which when cold drawn, exhibit, by characteristic X-ray patterns, molecular orientation along the fiber axis. The properties of such film may be further enhanced by a post casting treatment of the type disclosed by Alles, U.S. Pat. No. 2,779,684 to provide a highly dimensionally stable polyester film suitable for use in the photographic industry.
Traditionally, the casting procedure for such film involves extruding molten polymer onto a casting wheel at a temperature sufficiently high to assure good flow properties for the extrudate during the extrusion process, and then quenching said extrudate to provide a self-supporting web of film, which is further treated as shown in Alles above. The purpose of the quenching process is to cool the molten polymer to a temperature below its glass transition temperature through the bulk of the cast web to form a permanently self-supporting web which may be then stored or subjected to other treatment as desired. Commonly the casting wheel upon which the molten polymer is extruded also serves to quench the film. To that end, the casting wheel is hollow and a coolant fluid is circulated within said wheel to remove heat from the surface of the web supported on the wheel. Simultaneously the outer surface of the cast web may also be cooled, using air impingement, cooling baths, chilled rolls, or combinations thereof. U.S. Pat. No. 3,853,447 discloses a combination casting and quenching apparatus which is useful in providing a high quality quenched web by employing a combination of the aforementioned cooling techniques.
While the use of combined casting and quenching wheels has proven satisfactory for the production of high quality polyester film, problems begin to surface when higher and higher production rates are desired. Since heat must be removed from the molten polymer while on the casting wheel, either (1) the size of the wheel must be increased as the production rate is increased, or (2) heat must be removed from the polymer at a faster rate, or both. At the same time, the casting wheel must be maintained true to a high degree of accuracy since it is the distance between the extruder lip and the outer wheel surface that determines the final product thickness and, even more important, the thickness uniformity of the cast web. The casting wheel is subjected to substantial stresses because of temperature fluctuations ranging from (a) over 275.degree. C. at the point where the molten polymer contacts the wheel surface, to (b) below 75.degree. C. at the point where the polymer has been cooled to below its glass transition temperature, to (c) the coolant fluid temperature of about 15.degree. C. In addition to the above, an axial temperature gradient tends to form on the casting surface because the extreme ends do not carry any polymer and are exposed to the atmosphere and consequently to room temperature. The resulting distortion of the casting surface can be fatal to the success of the casting operation, a problem which is discussed in Pabo, U.S. Pat. No. 3,142,866. To prevent such distortion or deformation it is common practice to go to casting wheels of increased size and massive construction or adopt complicated wheel structures as shown in Pabo above. However, the coolant fluid can only remove heat at a finite rate from the interior surface of the casting wheel. Thus, the rate of production as measured in terms of pounds of polymer per hour for a given installation appears limited by engineering factors related to the casting wheel and coolant heat-removing ability.